JOHANNESBURG, Sept 17 (Reuters) - South African power utility Eskom said on Monday that it had fewer than 20 days of coal supplies at 10 of its 15 coal-fired power stations, posing a threat to national power supplies.

A company spokesman said a major supplier, owned by the Gupta family, had cut supplies as it seeks insolvency protection and Eskom was quickly trying to secure new contracts with other companies to ensure it had enough coal.

Cash-strapped Eskom is critical to Africa’s most industrialised economy as it supplies more than 90 percent of the country’s power, predominantly by burning coal. It is also one of South Africa’s most indebted state firms.

“Out of our 15 coal-fired power stations, 10 of them have less than 20 days. Clearly this is contrary to what the regulator has prescribed,” Eskom spokesman Khulu Phasiwe said.

Eskom, which is battling to recover from a financial and leadership crisis, said it will transport coal from other power stations to those facing constraints and is in discussions with 12 coal suppliers to secure coal contracts.

“If there is no coal it may hamper our ability going forward, but to avoid it being hampered we are diverting coal from other power stations,” Phasiwe said.

The main reason for the coal shortages is that Tegeta Exploration and Resources, a coal-mining firm owned by the Gupta family, has cut supplies to Eskom as it seeks insolvency protection, Phasiwe said.